1. Field of the Invention
The present invention relates to a polarization phase difference plate and, particularly, to a polarization phase difference plate utilizing structural birefringence caused due to the fine structure.
2. Description of the Related Art
The polarization phase difference plate enables conversion of polarization state of incident light by birefringence. As such polarization phase difference plate, λ/4 plates, λ/2 plates and the like are well known.
The polarization phase difference plates can be roughly classified into following three types.
The first type is a polarization phase difference plate formed by stretching an optical polymer film that is optically uniaxial in such a manner that a total amount of birefringence (described in detail later) for the wavelength λ of the light to be used becomes λ/4 (90°) or λ/2 (180°).
The second type is a polarization phase difference plate formed in such a manner that a total amount of birefringence for the wavelength λ of the light to be used becomes λ/4 (90°), by adjusting the film thickness through optically polishing uniaxial crystals such as quartz crystals.
The third type is a polarization phase difference plate that utilizes structural birefringence generated due to the fine periodic structure.
All of those three types of polarization phase difference plates are used as elements for converting polarized light.
Specifically, those are used to convert linearly polarized light into circularly polarized light by directing the light of the linear polarization in the direction at 45° with respect to the fast axis (advance phase axis) or the slow axis (delay phase axis) of the polarization phase difference plate. Inversely, those are used to convert the circularly polarized light into the linearly polarized light by directing the circular polarization towards the polarization phase difference plate.
The aforementioned structural birefringence will now be described in detail. The structural birefringence is known as a phenomenon where birefringence is generated from an anisotropic-shaped one-dimensional periodic structure in a region about a half the wavelength of the light or smaller.
As an example of such structural birefringence, let us look into the case where, as shown in FIG. 1, for example, there periodically exist two kinds of media, i.e. a medium (an air layer) having permittivity ∈1 and a medium (convex part 2 of diffraction grating 1) having permittivity ε2 particularly in a minute-size region with only zero-order diffraction gratings present, and light makes incidence in the z-axis direction (in the longitudinal direction of FIG. 1).
It is known in this case that average permittivity in the x-axis direction (in the lateral direction of FIG. 1) and in the z-axis direction can be expressed by following expressions based on ideas of an effective refractometry.∈∥(0)=(1−f)·∈1+f·∈2 (filling factor f=w/Λ)  (1-1)1/∈⊥(0)=(1−f)/∈1+f/∈2  (1-2)Each of expressions (1-1) and (1-2) is applicable on the assumption that the media spread infinitely in the x-axis and y-axis directions. Further, each expression applies approximately on the assumption that the periods Λ of each medium with the permittivity of ∈1, ∈2 (see FIG. 1) are much smaller than the wavelength of the light. f in the expression (1-1) is a parameter called the filling factor, which is expressed as w/Λ, the ratio of the size of the medium (convex part 2) with the permittivity ∈2 in the x-axis direction with respect to the period Λ.
The square of diffractive index n is the permittivity ∈. Further, birefringence amount Δn, i.e. the amount of the birefringence generated per unit height of the polarization phase difference plate (in other words, the polarization phase difference amount) is proportional to the amount of ∈∥(0)−∈⊥(0) in the above-described expressions. Further, the amount of birefringence generated in the entire polarization phase difference plate, i.e. the total amount of birefringence, is roughly proportional to the depth d of the diffraction grating 1 (in other words, the height of the convex part 2).
As can be seen from each of the expressions (1-1), (1-2), use of uniaxial medium that utilizes the structural birefringence provides such an advantage that the birefringence amount Δn and, moreover, the total amount of the birefringence can be controlled artificially by changing the value of the filling factor f of the periodic structure of the diffraction grating.
Regarding this, there is described in detail in Optical Review vol. 2 (1995) pp. 92–99. It is known that precise birefringence amount Δn and total amount of the birefringence can be calculated almost accurately by RCWA method (Rigorous Coupled Wave Analysis) that is one of the rigorous electromagnetic analyzing methods.
In order to minimize the loss of light in the polarization phase difference plate having such structural birefringence, it is necessary for the period Λ to be in such a size that high-order diffraction light is not generated.
As an expression of a condition for not generating the light of first order or higher, the following expression (2) needs to be satisfied.(Λ/λ)<1/(max[ns, ni]+nj·sin θmax)  (2)where, Λ is a period of the diffraction grating that constitutes the polarization phase difference plate, λ is the wavelength of the light to be used, and θ is an incident angle of the light with respect to the polarization phase difference plate. Furthermore, in the expression (2), ns is the diffractive index of the base material that constitutes the polarization phase difference plate, and ni is the diffractive index of the medium (air) on the incident side.
Assuming that the wavelength of the light to be used is 650 nm, the diffractive index is 1.512, and the incident angle is 0°, the period Λ becomes smaller than 0.429 μm.
That is, it is found that the period needs to be smaller than 429 nm in order to form the polarization phase difference plate with less light loss.
There have been proposed various kinds of polarization phase difference plates so far, which utilize such type of structural birefringence.
For example, Patent Literature 1 discloses, as a polarization phase difference plate, a wave plate 4 utilizing a lamellar-shape diffraction grating 1 of a sub-wavelength region as shown in FIG. 2.
[Patent Literature 1] Japanese Patent Unexamined Publication 2003-207636
[Patent Literature 2] Japanese Patent Unexamined Publication 2005-99099
[Patent Literature 3] Japanese Patent Unexamined Publication 2005-44429
However, there are problems in the conventional polarization phase difference plate in terms of its optical properties or manufacture as described below.
That is, conventionally, Fresnel reflection is caused on the surface of the polarization phase difference plate in the thickness direction due to a difference between the diffractive index of the diffraction grating constituting the polarization phase difference plate and the diffractive index of the air layer. This Fresnel reflection increases the power loss of the transmission light that transmits the polarization phase difference plate.
Furthermore, conventionally, interference (Fabry-Perot multiple interference) is generated between the Fresnel reflection on the surface of the polarization phase difference plate in the thickness direction and the Fresnel reflection on the other surface of the polarization phase difference plate in the thickness direction. Therefore, particularly in the case where the light to be used is light with high interference such as laser beams, the intensity of the transmitting emission light becomes oscillated, thus causing it to be unstable.
These problems are factors for deteriorating the quality of the polarization phase difference plate.
As a method for eliminating such Fresnel reflection, there is a method as disclosed in Patent Literature 2, for example, which perform coating of an antireflection layer made of a dielectric multilayer film on the surface of the polarization phase difference plate (wave plate in Patent Literature 2) using vacuum deposition or the like. In this case, the dielectric multilayer film can be formed by alternately laminating a high-diffractive layer and a low-diffractive layer such as SiO2, TiO2.
However, when employing such antireflection film, there is required a device for coating the antireflection film in vacuum. Thus, it is disadvantageous in terms of the cost.
Furthermore, many of the cases use an inorganic substance such as oxide for the dielectric multilayer film so that, particularly when the polarization phase difference plate is of high polymer, adhesion between the antireflection coating and the polarization phase difference plate tends to be weak. In addition, due to a difference between the thermal expansion coefficients, there may cause manufacture inferior such as separation of the antireflection film, cracks, etc.
In the art disclosed in Patent Literature 3, the top end of the fine periodic structure is formed in a taper shape for reducing Fresnel reflection on the surface of the wave plate. However, there is no mention of parameters for specifying the taper shape and taper angle which are effective for reducing the Fresnel reflection. Further, the wavelength band that is effective for antireflection is not mentioned either.